The present invention relates generally to the field of dentistry. More specifically, the present invention relates to a microwave polymerization system for dentistry that utilizes specifically controlled microwave energy to cure polymer materials so as to produce dental prosthetics and dental composites having improved physical characteristics.
The use of polymer materials in the dental arts for the restoration of lost or damaged teeth is well known. Such uses fall into two general categories: (i) the use of polymer materials to produce dental prosthetics, such as dentures, bridges and crowns, that are either permanent or removable articles, and (ii) the use of polymer materials to create dental composites for fillings to repair teeth instead of using conventional amalgam fillings or as veneers to refinish tooth enamel surfaces. The first category of dental articles, dental prosthetics, are created outside of the patient (i.e., extra-oral), typically by making an impression of what the desired article should look like and then molding the article to match the impression. The second category of dental articles, dental composites, are created directly in the patient""s mouth (i.e., intra-oral) as fillings or veneers to repair or resurface teeth. Regardless of which category is being considered, dental articles made of polymer materials must have adequate strength, durability, and dimensional stability and must also be biocompatible and chemically inert. It is also important to be able to process each type of dental article rapidly, conveniently, safely and economically.
An example of a dental prosthetic in the first category of dental articles that is created using polymer materials is a removable denture. Most commercial dentures are created using a paste or resin matrix formed of various polymers, co-polymers and monomers (typically methyl methacrylate), as well as certain cross-linking agents, initiators, accelerators and other additives. This resin matrix is formed into a plaster mold and is then hardened or cured by applying energy in the form of heat. Typically, the plaster mold containing the uncured denture is prepared in a dental laboratory based on an impression taken by the dentist. To cure the resin matrix, the plaster mold is placed into a flask that is then put in a thermal water-bath for up to 8 hours. This conventional process of curing a denture takes such a long time because both the plaster molds and the polymers in the resin matrix are relatively poor thermal conductors and are heated only from the outside via the thermal water-bath. The conventional process can also result in large numbers of voids and significant shrinkage during curing due to uneven thermal conduction and non-uniform polymerization of the resin matrix. These problems are discussed in Feilzer A. J. et al., xe2x80x9cCuring contraction of composites and glass-ionomer cements,xe2x80x9d Journal of Prosthetic Dentistry, Vol. 59, pp. 297-300 (1988); and Ferracane J. L. et al., xe2x80x9cWear and marginal breakdown of composite with various degrees of cure,xe2x80x9d J Dent. Res., Vol. 76, No. 8, pp. 1508-16 (1997). The lack of completely uniform polymerization of the denture also leaves residual monomers that are toxic and can act as irritants to oral tissues and compromise the physical characteristics of the denture.
In an effort to overcome the long cure times associated with the conventional thermal water-bath technique, a technique of using commercial microwave ovens to heat and cure polymer resins to form dental prosthetics has been developed. In the conventional thermal curing method for polymer dental articles, a temperature differential is required to force heat by conduction from the surface of the flask and mold to the center of the article. Because the heat penetrates from the outside to the internal portions of the material by thermal conduction, overheating and degrading polymers can occur at the outer surface of the article. When microwaves are used to initiate the thermal curing processing, it is possible for the article to be heated uniformly as the electromagnetic radiation instantaneously penetrates deeply and heating occurs throughout all three dimensions of the irradiated article. The main advantages provided by microwave energy include a rapid internal heating independent of the heat flow through the surface, as well as minimal thermal lag and thermal gradients throughout the interior of the article, which results in a more homogeneous curing of the article with a higher degree of conversion of monomers into polymer chains.
Comparisons of these two techniques can be found in Hayden W. J., xe2x80x9cFlexure strength of microwave-cured denture baseplatesxe2x80x9d, General Dentistry, Vol. 343, pp. 367 (1986); Al Doori D et al. xe2x80x9cA comparison of denture base acrylic resins polymerized by microwave irradiation and by conventional water bath curing systems,xe2x80x9d Dental Materials, Vol. 4, pp. 25-32 (1988); and Geerts G. et al., xe2x80x9cA comparison of the bond strengths of microwave and water bath-cured denture materials,xe2x80x9d The Journal of Prosthetic Dentistry, Vol. 66, No. 3, pp. 403-07 (Sep. 1991). Various types of flasks and molding equipment that can be used in conjunction with a commercial microwave oven for processing and curing dental articles made of polymers have been developed as described, for example, in U.S. Pat. Nos. 4,971,735, 5,151,279, 5,324,186 and 5,510,411, European Patent No. 0 687 451 A2, and Japanese Patent No. JP7031632A. The repair of dentures and related articles using microwave processing is also described in Turck M. D. et al, xe2x80x9cMicrowave processing for dentures, relines, repairs and rebases,xe2x80x9d The Journal of Prosthetic Dentistry, Vol. 69, No. 3, pp. 340-43 (1993). Generally, dentures cured by commercial microwave ovens have improved mechanical properties, and often have better adaptation than those cured by the water-bath method. The primary advantage of microwave curing, however, is the reduced processing times which can be shortened from 8 hours or more to as little as a few minutes.
While the use of commercial microwave ovens to cure dental prosthetics solves some of the problems of conventional thermal water-bath cured prosthetics, dental prosthetics processed in this manner can be less than satisfactory in terms of their physical and biocompatibility characteristics because varying degrees of cure, micro-shrinkage and porosities are still present. Any large degree of micro-shrinkage or porosities in the polymers of dental prosthetics cured using conventional microwave ovens will lead to fitting inaccuracy and unreliability. These problems are discussed in Wallace P. W. et al., xe2x80x9cDimensional accuracy of denture resin cured by microwave energy,xe2x80x9d The Journal of Prosthetic Dentistry, Vol. 68, pp. 634-40 (1992); and Salim S. et al. xe2x80x9cThe dimensional accuracy of rectangular acrylic resin specimens cured by three denture base processing methods,xe2x80x9d The Journal of Prosthetic Dentistry, Vol. 67, pp. 879-85 (1992).
It understood in the dental arts that micro-shrinkage is primarily due to the resin matrix. The physical and mechanical properties of a polymer material, such as hardness, stiffness and abrasion resistance and strength, are highly influenced by the arrangement of the resin matrix when the fillers and coupling agents are fixed during the curing process. Micro-shrinkage results from the shorter distance between atoms in the resin matrix after polymerization than before polymerization. The monomers in the resin matrix are located at Van der Waals distance, which change to a covalent bond distance once the resin matrix is polymerized. If all of the monomers in the resin matrix are not converted into polymer chains during the polymerization curing process, then this change in distance can induce mechanical stresses in the form of micro-shrinkage in those areas where there was not complete conversion of the monomers into polymers. Commercial resin matrices are found to undergo volume shrinkage of as much as 7% with most resin matrices shrinking 2-3%. This kind of micro-shrinkage causes volumetric dimensional change that can result in poor fitting of the dental prosthetic to oral tissues and can build up mechanical stress in the dental prosthetic that can lead to premature mechanical failure. A discussion of some of these issues can be found in D. Bogdal, xe2x80x9cApplication of Diol Dimethacrylates in Dental Composites and Their Influence on Polymerization Shrinkage,xe2x80x9d J. Appl. Polym. Sci., Vol. 66, pp. 2333-2337 (1997), and D. Bogdal et al., xe2x80x9cThe Determination of Polymerization Shrinkage of Materials for Conservative Dentistry,xe2x80x9d Polimery, Vol. 41, pp. 469 (1996).
Another problem caused by the residual monomers not being converted into polymer chains during the polymerization curing process is the leaching of any unbound monomers or additives out of the article. The leaching has an impact on both the structural stability and biocompatibility. The residual monomers can leach into salivary fluids which then irritates any mouth tissues in contact with these contaminated fluids; or the residual monomers can diffuse directly into the dentin and pulp of teeth adjacent to the dental prosthetic. These problems are described in Ferracane J. L., xe2x80x9cElution of leachable components from composites,xe2x80x9d Journal of Oral Rehabilitation, Vol. 21, pp. 441-52 (1994); and Hume W. R. et al., xe2x80x9cBioavailability of components of resin-based materials which are applied to teeth,xe2x80x9d Crit. Rev. Oral Biol. Med., Vol. 7, No. 2, pp. 172-79 (1996).
The primary solutions to these problems have focused on increasing the degree of polymerization and cross-linking of all of the monomers in the resin matrix by changing the formulation of the resin matrix. Improvements in resin formulation involve, for example, the introduction of spiro orthocarbonates and stereoisomers. U.S. Pat. No. 5,502,087 describes various polymer-based resins that are designed to improve the physical characteristics of thermal water-bath cured resin matrices or light-activated resin matrices. U.S. Pat. No. 5,147,903 describes polymer materials that exhibit desired degrees of swelling and cross-linking for light-activated resin matrices. Examples of polymer resin matrices that are specifically formulated to utilize microwave energy supplied by a commercial microwave oven for the thermal polymerization of the polymers into dental articles are shown in U.S. Pat. Nos. 4,873,269, and 5,218,070, and Canadian Patent No. 2,148,436. The impact of the role played by the polymer initiator in a microwave cured resin matrix has been evaluated by Urabe H. et al. in xe2x80x9cInfluence of polymerization initiator for base monomer on microwave curing of composite resin inlays,xe2x80x9d Journal of Oral Rehabilitation, Vol. 26, pp. 442-46 (1999).
Another solution to these problems is also described in Canadian Patent No. 2,148,436 in which the resin matrix in the mold is compressed by a mechanical ram that injects additional uncured polymer components into the mold while the mold and flask are being cured inside a commercial microwave oven. The mechanical ram slowly forces some of the uncured polymer material contained in an injection cartridge through a passageway or sprue into the mold. The addition of polymer material applied under mechanical compression while it is still fluid is another way of reducing problems related to the polymerization shrinkage and porosity. U.S. Pat. Nos. 5,175,008 and 5,302,104 and Canadian Patent No. 2,120,880 describe various solutions for providing similar types of mechanical compression in the context of molding dental prosthetics without using microwave energy to cure the resin matrix.
There had been relatively little research, however, into the potential impact of the microwave energy itself on the polymerization process. The most common use of microwave energy to cure dental prosthetics actually involves a two-stage process where, as described in U.S. Pat. No. 4,971,735, the microwave energy first quickly heats water in or around the flask or humidity in a moist mold, with the superheated water vapor then thermally conducting the generated heat to the resin matrix. Due to the superheated nature of this process, cure times can be dramatically reduced. Moreover, because the microwave energy is primarily being absorbed by water as the intermediary thermal agent, this process lends itself very well to the use of commercial microwave ovens operating at full power settings where the primary objective is to heat the water, and not necessarily the resin matrix. This is advantageous because commercial microwave ovens are controlled by cycling the microwave generator, known as a magnetron, on and off to provide an average output power that corresponds to the percentage of the duty cycle. For example, a 50% duty cycle operates the magnetron on 50% of the time and produces a power output in terms of watts of energy produced by the oven that would be one-half of the maximum power output of the oven.
The other mechanism by which microwave energy can be used to cure dental prosthetics involves a single stage process where the microwave energy is directly absorbed by the molecules of the resin matrix without any substantial assistance of an intermediary thermal transfer agent, such as water vapor. In this case, the microwave energy essentially vibrates the resin molecules in a complicated process that is dependent upon the specific nature of the chemical composition of the resin matrix. It has been found that where water vapor in the form of humidity is present in the process, the actual polymerization of the resin matrix will occur as a result of a combination of thermal conduction from water vapor and internal microwave energy transfer.
Unfortunately, the high temperatures generated in the targeted article by microwave heating with available commercial microwave ovens set to manufacturers"" recommendations (e.g., 3 minutes at 550W at 100% duty cycle for a G. C. Acron dental microwave oven) tend to result in the thermal degradation of, and porosity formation in, many thermosetting polymer materials since high temperatures (above 150xc2x0 C.) are often produced during these curing processes. In addition, hot and cold spots are often found within commercial microwave ovens that tend to create thermal gradients corresponding to these variations in microwave energy internal to the article being cured. The problems caused by these hot and cold spots can be compounded by the superheated nature of the water vapor, which effectively amplifies any uneven distribution of the thermal energy to the resin matrix.
What little research has been done on the effect of microwave energy on the polymerization process has generally focused on the duty cycle used for the microwave curing process. The impact on porosity of denture material cured using lower wattage, longer duration microwave cure times (i.e., a lower duty cycle for a longer time) versus higher wattage, shorter duration microwave cure times (i.e., a higher duty cycle for a shorter time) is compared in Alkhatib M. B., et al. xe2x80x9cComparison of microwave-polymerized denture base resins,xe2x80x9d The International Journal of Prothodontics, Vol. 3, No. 2, pp. 249-55 (1990). European Patent No. 0 193 514 B1 describes a microwave processing system for dental prosthetics that has a magnetron, a waveguide, a surface radiating antenna, a flask, and a temperature sensor that is inserted in the flask and connected to a regulating processor. The regulating processor limits the temperature in the flask as measured by the temperature sensor by turning on and off the magnetron based on frequency modulation of the duty cycle. Although not used for polymerization of dental articles, U.S. Pat. No. 5,645,748 does describe a microwave system for sterilization that controls duty cycle of a microwave oven for the purpose of minimizing arcing caused by metallic surgical or dental instruments.
Any increase in the degree of conversion of monomers into polymer chains in the polymerization process will result in improved mechanical properties and biocompatibility of microwave cured dental prosthetics. While existing solutions utilizing improved resin compositions and mechanical compression during the curing process with a commercial microwave oven have resulted in many improvements over the conventional thermal water-bath method of producing dental prosthetics, it would be desirable to further improve the uniformity and the degree of conversion of monomers into polymer chains in the polymerization process and further compensate for volumetric shrinkage during the polymerization process in order to produce even better dental prosthetics.
With respect to the second category of dental articles created using polymer materials, dental composites formed of polymer matrix-composites are increasingly being used as an alternative to mercury-containing dental amalgam for aesthetic and restorative dental materials. These kinds of polymer matrix-composites are usually photo polymerizable in that they are cured using some kind of light instead of heat. Generally, the polymer matrix-composite is based on a photo polymerizable polyfunctional methacrylate compound that can be used alone or as mixture with monomethacrylates, light sensitive cure initiators pigments and fillers in a mixture with various comonomers such as triethyleneglycol dimethacrylate. Although the half-life of these polymer matrix-composites cured by light is on the order of 5-8 years and, therefore, they tend to wear out earlier than conventional dental amalgams, the enhanced biofunctionality and more pleasing aesthetic qualities of these polymer matrix-composites have gained favor over conventional dental amalgams.
The main deficiencies of polymer composite resins used as dental composites are surface degradation that leads to inadequate wear resistance, polymerization shrinkage and a lack of density. In addition to the problems previously described for dental prosthetics, micro-shrinkage of polymer dental composites produces interfacial gaps on the surface of the composites, which can results in microleakage through the dental composite. The long-term consequence of such microleakage can be bacterial penetration into the tooth that can cause a variety of adverse reactions in the tooth such as pulp damage, tooth sensitivity, possible pulpal death and loss of adhesion of the dental composite.
Just as with polymer dental prosthetics, improving the degree of polymerization of polymer matrix-composites is generally considered to be one way of improving their physical and biofunctionality characteristics of polymer dental composites as this would lead to stronger dental composites that are less susceptible to degradation, wear and fracture. It would also lead to improved biocompatibility, since there would be reduced amounts of uncured monomer that could act as a biohazard.
Unlike polymer dental prosthetics, however, the curing of polymer matrix-composites by application of thermal energy generally has not been used to date. Obviously, in the case of the conventional thermal water-bath process, it would be impractical to require a patient to remain at the dentist""s office for up to 8 hours with their mouth open and with a tooth immersed in a hot water bath in order to set a thermally polymerizable matrix-composite. It is also not possible to place a patient""s mouth into a commercial microwave oven to set a thermally polymerizable matrix-composite.
While there are numerous hand-held medical catheter devices that utilize radio frequency and microwave energy to perform ablations and similar heating operations, for example, in the vascular system of a patient, there have been relatively few uses of thermal or electrical energy applied to hand-held dental tools for intra-oral applications. There have been a few hand-held dental probes that utilize an electrically resistive heated tip for diagnosis of dental decay or for melting a sealing material in an intra-oral context as described, for example, in U.S. Pat. Nos. 4,527,560 and 5,893,713. U.S. Pat. No. 5,421,727 describes the use of radio frequency/microwave energy as part of a hand-held endodontic root canal device to raise the temperature of the adjacent tooth, thereby tending to disinfect the tooth during the root canal procedure as a result of the increased temperature.
The extra-oral use of microwave energy for the purpose of characterizing dental decay in extracted teeth has been described by N. Hoshi et al in xe2x80x9cApplication of Microwaves and Millimeter Waves for the Characterization of Teeth for Dental Diagnosis and Treatment,xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, June 1998, Vol. 46, No. 6, pp. 834-38. This study confirmed the higher absorbency behavior of carious lesions in extracted teeth when irradiated by microwave energy as compared to the lower absorbency of such microwave energy by healthy enamel and dentin.
While existing photo polymerizable dental composites have enjoyed success as compared to conventional dental amalgams for dental fillings and veneers, it would be desirable to further improve the uniformity and degree of conversion of monomers into polymer chains in the polymerization process in order to produce even better dental composites.
The present invention is a microwave polymerization system for dentistry that utilizes specifically controlled microwave energy to cure polymer materials so as to produce dental prosthetics and dental composites. Unlike the microwave energy delivered by commercial microwave ovens which is controlled by pulsing a maximum output power on and off at a given duty cycle, the present invention utilizes metered and controlled microwave energy that is preferably continuous and voltage controlled, and regulates the application of this microwave energy by use of various feedback mechanisms. The metered and controlled microwave energy enables a higher degree of conversion and cross-linking of monomers into polymer chains in the polymerization process, thereby enhancing the physical and biocompatibility characteristics of both dental prosthetics and dental composites made in accordance with the present invention. In an extra-oral embodiment, gaseous pressure is applied to the resin matrix during the polymerization process to further enhance the polymerization process. In an intra-oral embodiment, the polymerization process can be accomplished with less overall energy and with composite-matrices that maximally absorb the microwave energy so as to reduce heating of adjacent tissue.
In one embodiment, a microwave oven is designed to apply continuous microwave energy in accordance with the extra-oral embodiment of the present invention for use in producing dental prosthetics at either a dental laboratory or a dental office. Microwave energy of between 1 GHz to 100 GHz, and preferably about 2.45 GHz, is continuously generated in the microwave oven in response to precisely controlled voltages of between 25 V and 110 V, depending upon the desired curing time and the particular composition of the resin matrix to be cured. A flask for use in the microwave oven is preferably provided with a mechanism to rotate the flask and with quick disconnect rotatable couplers for both liquid polymer insertion and gas pressurization while the article is rotating and undergoing the microwave curing process. The insertion of additional polymer and the gas pressurization system are utilized to maintain controlled gaseous pressure on the polymer material during the curing process to increase the density of the cured dental prosthetic and to compensate for micro-shrinkage that may occur during polymerization. Pressurization rates depend upon the strength characteristics of the polymer composition being used and preferably range between 10 psi to 125 psi with optimal ranges of between 12-35 psi. The flask may be equipped with an internal membrane to compress and adapt the pasty curable resin matrix onto the mold and with a vacuum forming system to draw the curable resin matrix into the mold and assist in maintaining the resin matrix in the mold during the curing process. In one embodiment, a cartridge is provided with quick disconnect couplers between the gas pressurization system and a sprue connected to the flask to permit filling of the mold with the curable resin matrix stored in the cartridge. Optionally, the microwave oven may be provided with features that also allow it to be used to sterilize dental prosthetics and other objects in a dental office or dental laboratory.